General Design Principles for DuPont Engineering Polymers - Module

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Effects of Time on Joint Strength As previously stated, a press-fit joint will creep and/or stress relax with time. This will reduce the joint pressure and holding power of the assembly. To counteract this, the designer should knurl or groove the parts. The plastic will then tend to flow into the grooves and retain the holding power of the joint. The results of tests with a steel shaft pressed into a sleeve of Delrin ® acetal resin are shown in Figures 10.09 through 10.11. Tests were run at room temperature. Higher temperature would accelerate stress relaxation. Pull out force will vary with shaft surface finish. Figure 10.09 Time vs. joint strength—2% interference Pull-out Force, N 3,000 2,000 d = 10 4 3 1,000 2 1.5 20 0 1 10 100 103 104 105 0 Time, h Figure 10.10 Time vs. joint strength—3% interference Pull-out Force, N 3,000 2,000 4 3 2 1,000 1.5 0 0 1 D Steel Shaft Delrin® Sleeve 2% interference Ratio D/d = 1.5 2 3 4 10 100 10 3 10 4 10 5 Time, h 3% interference Ratio D/d = 1.5 2 3 4 72 Figure 10.11 Time vs. joint strength—4 and 5% interference—Delrin ® Pull-out Force, N 3,000 2,000 1,000 4 1.5 0 0 1 10 Interference: 4% 5% Ratio D/d = 1.5 2 3 4 3 2 100 10 3 10 4 10 5 Time, h Snap-Fits The two most common types of snap-fits are: (1) those with flexible cantilevered lugs (see Figure 10.22) and (2) those with a full cylindrical undercut and mating lip (see Figure 10.12). Cylindrical snap-fits are generally stronger, but require greater assembly force than cantilevered lugs. In cylindrical snap-fits, the undercut part is ejected by snapping off a core. This requires deformation for removal from the mold. Materials with good recovery characteristics are required. For molding complex parts, cantilevered lugs may simplify the molding operation. Figure 10.12 Typical snap-fit joint d Return angle Cylindrical Snap-Fit Joint Return angle e Lead angle Lead angle Steel Shaft Delrin® Sleeve Table 10.05 Dimensions Cylindrical Snap-Fit d D (max., mm) e (mm) mm Delrin ® Zytel ® 101 Delrin ® Zytel ® 101 2 5 0.05 3 8 0.07 4 10 12 0.10–0.15 0.12 5 11 13 0.12–0.18 0.16 10 17 20 0.25–0.35 0.30 15 22 26 0.35–0.50 0.45 20 28 32 0.50–0.70 0.60 25 33 38 0.65–0.90 0.75 30 39 44 0.80–1.05 0.90 35 46 50 0.90–1.20 1.05 d D
Undercut Snap-Fits In order to obtain satisfactory results, the undercut type of snap-fit design must fulfill certain requirements: • Uniform Wall Thickness It is essential to keep the wall thickness constant throughout. There should be no stress risers. • Free to Move or Deflect A snap-fit must be placed in an area where the undercut section can expand freely. • Shape For this type of snap-fit, the ideal geometric shape is a circular one. The more the shape deviates from a circle, the more difficult it is to eject and assemble the part. Rectangular shaped snap-fits do not work satisfactorily. • Gates—Weld Lines Ejection of an undercut from the mold is assisted by the fact that the resin is still at a very high temperature, thus its modulus of elasticity is lower and elongation higher. This is not the case, later, when the parts are being assembled. Often an undercut part will crack during assembly due to weak spots produced by weld lines, gate turbulence, or voids. If a weld line is a problem and cannot be avoided by changing the overall design or by moving the gate to some other location, the section at the weld line can be strengthened by means of a bead or rib. Force to Assemble During assembly, cylindrical snap-fit parts pass through a stressed condition due to the designed interference. The stress level can be calculated following the same procedure outlined in the previous section on press fits. With snap-fits, higher stress level and lower design safety factor is permissible due to the momentary application of stress. The force required to assemble and disassemble snapfit parts depends upon part geometry and coefficient of friction. This force may be divided arbitrarily into two elements: the force initially required to expand the hub, and the force needed to overcome friction. As the beveled edges slide past each other, the maximum force for expansion occurs at the point of maximum hub expansion and is approximated by: F e = (1 + f) Tan (n) S d π D sL h W where: F e = Expansion force, kg (lb) f = Coefficient of friction n = Angle of beveled surfaces S d = Stress due to interference, MPa (psi) D s = Shaft diameter, mm (in) W = Geometry factor L h = Length of hub expanded, mm (in) 73 Coefficients of friction are given in Table 7.01. The formulas for maximum interference, l d, and geometry factor, W, are given below. For blind hubs, the length of hub expanded L h may be approximated by twice the shaft diameter. Poisson’s ratio can be found in the product modules. I = S dD s ( W + μ h + l – μs ) W E h E s and W= 1+( D s) 2 D h 1–( D s) 2 D h where: I = Diametral interference Sd = Design stress Dh = Outside diameter of hub Ds = Diameter of shaft Eh = Tensile modulus of elasticity of hub Es = Modulus of elasticity of shaft μh = Poisson’s ratio of hub material μs = Poisson’s ratio of shaft material W = Geometry factor The force required to overcome friction can be approximated by: Ff = fSdDsLsπ W where: Ff = Friction force f = Coefficient of friction Sd = Stress due to interference Ds = Shaft diameter Ls = Length of interference sliding surface Generally, the friction is less than the force for hub expansion for most assemblies. Examples Suggested dimensions and interferences for snapfitting a steel shaft into a blind hub of Zytel ® nylon resin are given in Table 10.05. Terminology is illustrated in Figure 10.12. A return bevel angle of 45° is satisfactory for most applications. A permanent joint can be achieved with a return angle of 90° in which case the hole in the hub must be open at the other end. It is a good practice to provide a 30° lead-in bevel on the shaft end to facilitate entry into the hub. For certain applications the snap-fit area can be provided with slots as shown in Figure 10.13. This principle allows much deeper undercuts, usually at the sacrifice of retaining force. For parts which must be
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dDuPont Engineering Polymers Genera
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8—Gears . . . . . . . . . . . . .
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1—General Introduction This secti
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Prototyping the Design In order to
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2—Injection Molding The Process a
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As a general rule, use the minimum
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Figure 3.11 Cored holes Figure 3.12
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Figure 3.19 Mold-ejection of rounde
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• Zytel ® nylon resin—Parts of
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4—Structural Design Short Term Lo
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Form of section Area A d d R R 1 1
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Table 4.03. Formulas for Stresses a
Page 25 and 26: Form of bar; manner of loading and
Page 27 and 28: Figure 4.01 Creep Stress (S), MPa (
Page 29 and 30: Figure 4.05 Creep in flexure of Zyt
Page 31 and 32: Example 1 It there are no restricti
Page 33 and 34: Determine the moment of inertia and
Page 35 and 36: Figure 4.11 Stress curves The compu
Page 37 and 38: Rim Design Rim design requirements
Page 39 and 40: Figure 5.08 Chair Seat Zytel ® 71
Page 41 and 42: Cantilever Springs Tests were condu
Page 43 and 44: 7—Bearings Bearings and bushings
Page 45 and 46: debris is moved around continuously
Page 47 and 48: Table 7.01 Coefficient of Friction*
Page 49 and 50: Figure 7.15: Connecting rod spheric
Page 51 and 52: 8—Gears Introduction Delrin ® ac
Page 53 and 54: 9—Gear Design Allowable Tooth Ben
Page 55 and 56: Delrin ® 500. See the section on D
Page 57 and 58: Figure 9.06 Sizes of gear teeth of
Page 59 and 60: The following additional examples i
Page 61 and 62: In practice, the most commonly used
Page 63 and 64: Driving Gear Meshing Gear Table 9.0
Page 65 and 66: A full-throated worm gear is shown
Page 67 and 68: Worm Material Gear Material Possibl
Page 69 and 70: 10—Assembly Techniques Introducti
Page 71 and 72: For these materials, the finer thre
Page 73 and 74: Table 10.01 Self-Threading Screw Pe
Page 75: Figures 10.07 and 10.08 show calcul
Page 79 and 80: Cantilever Lug Snap-Fits The second
Page 81 and 82: 11—Assembly Techniques, Category
Page 83 and 84: Figure 11.03 Drill spindle position
Page 85 and 86: Inertia Welding By far the simplest
Page 87 and 88: Machines for Inertia Welding The pr
Page 89 and 90: This is not always easy, because ap
Page 91 and 92: The holder a, will have equal and o
Page 93 and 94: Calculations for Inertia Welding To
Page 95 and 96: If, for example, the angles of the
Page 97 and 98: Welding Double Joints The simultane
Page 99 and 100: Figure 11.37 Commercial bench-type
Page 101 and 102: Heat is generated throughout the pa
Page 103 and 104: Figure 11.45 Tapered or stepped hor
Page 105 and 106: Weld strength is therefore determin
Page 107 and 108: ) General Part Design The influence
Page 109 and 110: eticule in the eye piece is suitabl
Page 111 and 112: For this reason, if the strength of
Page 113 and 114: Figure 11.64A shows a variation whi
Page 115 and 116: 3. Vibrations, generated either by
Page 117 and 118: If one part is only vibrated at twi
Page 119 and 120: Another joint design, with flash tr
Page 121 and 122: Figure 11.82. A linear welded motor
Page 123 and 124: B) Commercial linear and angular we
Page 125 and 126: Hot Plate Welding of Zytel ® One o
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Figure 11.94 Applications of riveti
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Tolerances may be difficult to hold
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are used in either hand- or power-o
Page 133 and 134:
ather than scrape, these drills wil
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The polishing operation is performe
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